1. Field of the Invention
The present invention generally relates to solar cells and, more particularly, to the solar cells of a wrap-around type having no electrode appearing on a light receiving surface thereof, but having emitter and base electrodes on a backside surface opposite to the light receiving surface. The present invention also relates to a method of making such solar cells.
2. Description of the Prior Art
FIGS. 9A to 9D and FIGS. 10A to 10D illustrate the sequence of manufacturing VEST cells which are representative of prior art wrap-around solar cells disclosed by M. Deguchi et al., "A Novel Fabrication Technique for Low cost Thin Film Polycrystalline Silicon Solar Cells", Technical Digest of the 7th International Photovoltaic Science and Engineering Conference, p243 (1993). The wrap-around solar cells are known having a structure in which no electrode show up on a light receiving surface of each solar cell and emitter and base electrodes are disposed on a backside surface of the respective solar cell opposite to the light receiving surface.
FIGS. 9A to 9D are plan views of the VEST cells as viewed from the backside surface thereof whereas FIGS. 10A to 10D are cross-sectional views taken along the lines 10A--10A, 10B--10B, 10C--10C and 10D--10D in FIGS. 9A to 9D, respectively. As shown in FIGS. 9A and 10A, a thin silicon substrate 1 of a thickness not greater than 100 .mu.m and having first and second major surfaces opposite to each other is formed on the first major surface with a plurality of regularly spaced via-holes 3, followed by formation of an emitter layer 2 of a conduction type opposite to that of the silicon substrate 1 by the use of a thermal diffusion technique. In the illustrated example, the silicon substrate 1 has a p-type conductivity and, accordingly, the emitter layer 2 has a n-type conductivity.
Then, as shown in FIGS. 9B and 10B, the silicon substrate 1 having the via-holes 3 and the emitter layer 2 both formed thereon is affixed to a transparent glass plate 4 by the use of a transparent bonding material such as, for example, a transparent silicone resin 5 with the second major surface thereof oriented towards the glass plate 4. The glass plate 4 eventually forms a protective covering for solar cells. The amount of the transparent silicon resin 5 applied to secure the silicon substrate 1 to the glass plate 4 should be carefully controlled to avoid the possibility that the bonding material will undesirably run out of some or all of the via-holes 3. For this purpose, the amount of the transparent silicon resin 5 is so chosen as to correspond to the total volumes of the via-holes 3.
Thereafter, as shown in FIGS. 9C and 10C, a p-type electrode 6 and an n-type electrode 7 are disposed on the silicon substrate 1 and the emitter layer 2, respectively, followed by soldering tab electrodes 8 to the p-type and n-type electrodes 6 and 7, respectively, as shown in FIGS. 9D and 10D.
The prior art method of making the solar cells as discussed above requires utmost care to be taken during application of the transparent bonding resin 5. Unless the amount of the transparent bonding resin 5 is chosen so as to correspond to the total volumes of the via-holes 3, portion of the transparent bonding resin 5 will, when the glass plate 4 is deformed and/or the transparent bonding resin 5 is applied unevenly, run out of the via-holes 3 to spread over local surface areas of the silicon substrate 1 around the via-holes 3 as indicated by 5a in FIGS. 9B and 10B. The resin flashes 5 so formed are cured when and after the assembly is baked and subsequently solidified. Because of this, during formation of the electrodes as shown in FIGS. 9C and 10C that takes place subsequent to the application of the transparent bonding resin 5, the electrodes 6 and 7 may be partly formed on some or all of the resin flashes 5, accompanied by reduction in surface area of contact between the electrodes 6 and 7 and the silicon substrate 1. Reduction in surface area between the electrodes 6 and 7 and the silicon substrate 1 will in turn result in reduction in output characteristic of the resultant solar cells.
When it comes to a mass-production of solar cells, the use of a sputtering technique or a vapor deposition technique, both requiring the use of a vacuum chamber, will give rises to a relatively low throughput and, therefore, in order to increase the throughput, a metal paste printing technique to form the electrodes is essential.
However, according to the prior art method of making the solar cells, since a metal paste is applied to form the electrodes subsequent to solidification of the transparent bonding resin 5, material for the electrodes is required to be of a kind that can be sintered or baked at a temperature, for example, not higher than 300.degree. C. at which the transparent bonding resin 5 will not change in properties. For this reason, as compared with the material that is baked at a temperature higher than 600.degree. C. that is generally employed, the electrodes must have been of a kind having a relatively high resistivity and a relatively high contact resistance.
As discussed above, since in the practice of the prior art method of making the VEST cells the electrodes are formed subsequent to application of the silicon substrate 1 to the glass plate 4, the transparent bonding resin 5 must be uniformly applied to the glass plate 4 to an extremely limited thickness. Once the transparent silicon resin runs out onto areas where the electrodes are subsequently formed, the area of contact surface of the electrodes will decrease. In addition, the metal paste that can be employed for the electrodes is limited to the kind that can be sintered at a temperature sufficiently low to avoid any possible change in properties of the transparent resin 5.